Methods and apparatus for interferometric interrogation of an optical sensor

ABSTRACT

A high-speed interrogation system is provided for interferometric sensors, one example of which is an EFPI sensor, that operates based on spectral interference. The system uses a two mode operation that includes a lower speed, accurate absolute measurement mode and a higher speed, relative measurement mode. The system achieves greater overall measurement accuracy and speed than known sensor interrogation approaches.

PRIORITY APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/500,243, filed Jan. 30, 2017, which is the U.S. national phase ofInternational Application No. PCT/US2015/042439 filed Jul. 28, 2015,which designated the U.S. and claims the benefit of U.S. provisionalpatent application 62/030,793, filed on Jul. 30, 2014, the entirecontents of each of which are hereby incorporated herein by reference.

GOVERNMENT RIGHTS

This invention was made with Government support under Contract No.FA8650-14-C-2529 awarded by the United States Air Force. The Governmenthas certain rights to the invention.

BACKGROUND AND PROBLEM RECOGNITION

The technology in this application relates to optical measurementapparatus and techniques.

FIG. 1 shows a schematic block diagram of an interferometric sensorarrangement 10. An input-output fiber 11 (e.g., a single mode fiber)conducts light from a light source, e.g., a laser source 12, via acoupler 14 to an interferometric sensor 13 (e.g., an EFPI sensor ortransducer). A detector 16 detects light reflected back from the sensor13 via the coupler 14 over fiber 15 (e.g., a single mode fiber 15).

Extrinsic Fabry Perot Interferometer (EFPI) sensors are based on thechange in the optical length of a low-finesse Fabry-Perot cavity withrespect to an applied measurand. FIG. 2 shows an example EFPI straingauge sensor 10 that includes an optical fiber 11 inserted into one endof a silica capillary tube 21 used to add structure and prevent debrisfrom entering a cavity or gap 24 formed between the end of the opticalfiber and a reflective surface inserted into the other end of thecapillary tube. The cavity or gap 24 is formed between the flat endfaceof the transmitting fiber 11 indicated at reference reflection (R₁) anda sensing reflection (R₂) surface appropriate for the application, whichin this example is the surface of a fiber or reflective object 22 isinserted into the opposing end of the capillary tube 21.

The width of the Fabry-Perot cavity 14, referred to as a gap, ismeasured by interrogating the sensor 10 with a light source whichreflects off both the fiber endface (R₁) and the surface (R₂) of thetransducer 16 and interpreting the resulting interference pattern. Whenthe light arrives at the source fiber end-face, a portion is reflectedoff the interface caused by differing indices of refraction between thefiber and the transparent media (R1) and the remaining light propagatesthrough a cavity or gap with a second reflection occurring at themedia/fiber interface (R2). The distance between R1 and R2 is same asthe length of the gap and is one half of an optical path length. In aninterferometric sense, R1 is the reference reflection, and R2 is thesensing reflection. These reflective signals interfere constructively ordestructively based on wavelength and the optical path length differencebetween the reference and sensing fibers. Small movements fromenvironmental or other physical forces cause a change in the cavity orgap length causing a phase difference between the sensing and referencewaves producing interference patterns called “fringes.” The sensitivityto changes in gap length is proportional to the visibility of theinterference fringes reflected back into the input fiber. Exampleinterference fringes (intensity v. optical frequency) are shown in FIG.3A for a 60 μm gap and in FIG. 3B for a 120 μm gap; the opticalfrequency spacing of the interference fringes is inversely proportionalto the gap width.

EFPI technology may be used to monitor a wide variety of parameters invarious environments (including harsh environments) such as strain,temperature, pressure, shear, acceleration, electrical/magnetic field,radiation magnitude and radiation types, humidity, chemicalconstituents, and any other measurable parameters (sometimes called themeasurand). Example advantages of EFPI sensors include high temperatureoperation, small size, and immunity to electrical noise.

EFPI sensors may be analyzed using interrogation systems generallyfalling into one of two categories of systems: 1) high update rateinterrogation systems that make fast relative measurements but have lessabsolute accuracy, and 2) low update rate absolute interrogation systemsthat provide more accurate results using a wide range of wavelength datarequiring more extensive processing and interrogation time than the highupdate rate interrogation systems, which reduces the time betweenmeasurements.

Wide-spectrum optical emitters, which may be white-light sources such aslight emitting diodes (LEDs) or highly coherent light sources such asswept-wavelength lasers, can produce absolute measurements of theoptical path length of the Fabry-Perot cavity or gap. Interrogationsystems based on white-light optical sources launch a broad range ofoptical frequencies into the sensing fiber at the same time. Reflecteddata is collected with a spectrometer to produce a measurementinterference signal intensity vs. laser optical frequency. The speed ofan interrogation system based on a white-light optical source is limitedby the required integration time of the spectrometer, which is dependenton overall signal intensity and range and may typically be on the orderof 1 ms to produce a measurement.

EFPI sensors may also be interrogated using tunable laser sources withwide tuning ranges. In this scheme, the laser's optical frequency isswept across a range. As the laser's frequency changes, the interferencesignal's intensity varies sinusoidally. This sinusoidal signal iscollected at a photodetector and converted from the time domain to thespectral domain using the known rate of laser frequency sweep. The speedof data collection is limited by the sweep range and rate of the tunablelaser. The speed of an interrogation system based on typical high-speedtunable lasers may be approximately on the order of 50 ms to produce ameasurement.

Amplitude-based approaches can increase the speed of EFPI interrogation.Amplitude-based schemes do not directly generate wide wavelength rangesof spectral domain data. Using one or more narrowband lasers, anamplitude-based scheme monitors the change in reflected signalamplitude(s) vs. time using simple photodetectors. By simplifying thedetection scheme—the lasers are continuously emitting, and thephotodetectors are continuously collecting data—amplitude-basedinterrogators can achieve high update rates. A primary limitation ofmeasurement speed is the bandwidth of the photodetector and acquisitionelectronics.

A simple amplitude-based interrogator uses a fringe-counting scheme. Asingle, narrowband laser emits light at a fixed frequency forinterrogating an EFPI sensor. When the sensor's gap changes at aconstant rate, the intensity of the return signal varies sinusoidallywith time. Assuming a monotonic change in the measurand during the timeperiod of interest, the total change in the measurand may be determinedin part by counting the number of times the intensity reached a maximumvalue (the number of fringes produced by the total change in the gaplength) plus the fraction of the next fringe generated. This fringecounting method provides only a relative, rather than an absolute,measurement of gap displacement. In addition, the accuracy of thisrelative measurement suffers when the detected intensity is in thevicinity of a maximum or minimum. This is due to a lower rate ofintensity change with phase at the extrema of a sine wave as comparedwith the linear, rapidly changing regions of the waveform.

What is needed is a system that is capable of producing highly accurateabsolute measurements of an interferometric (e.g., EFPI) sensor whilealso obtaining high update rates.

SUMMARY

A high-speed interrogation system is provided for interferometricsensors, one example of which is an EFPI sensor, that operates based onspectral interference. The system uses a two mode operation thatincludes a lower speed, accurate, absolute measurement mode and a higherspeed, relative measurement mode. The system achieves greater overallmeasurement accuracy and speed than known sensor interrogationapproaches.

Example embodiments include measuring methods and apparatus for use withan interferometric sensor. In a first operational mode, light isprovided over a range of source wavelengths to the interferometricsensor. The intensity of the interference signal produced by theinterferometric sensor over the range of wavelengths is converted to anelectrical amplitude signal at the detector and is measured. An absolutemeasurement of an optical path length associated with theinterferometric sensor is determined based on the measured amplituderesponse over the source wavelength range. The optical path lengthvaries depending on one or more physical parameters to be measured usingthe interferometric sensor. In a second operational mode, light at afirst predetermined wavelength and at a second different predeterminedwavelength is provided to the interferometric sensor. The first andsecond wavelengths are chosen such that there is a predetermineddifference in the sensor interference fringe phase at the first andsecond wavelengths. A first amplitude response of an interferometricsignal produced by the interferometric sensor at the first predeterminedwavelength is measured. A second amplitude response of aninterferometric signal produced by the interferometric sensor at thesecond predetermined wavelength is measured. A difference between thefirst and second amplitudes is determined. A relative optical pathlength change based on the difference is determined. The absoluteoptical path length is combined with the relative optical path lengthchange to determine a current absolute optical path length. A signalcorresponding to the current absolute optical path length is generatedthat equates to a current sensor measurand value or from which a currentsensor measurand value is determined.

In example implementations, the second wavelength is modified so thatthe predetermined difference in the sensor interference fringe phase atthe first and second wavelengths is maintained. The modifying may bebased on the determined relative optical path length change, and thepredetermined phase in the sensor interference fringe phase is aquadrature phase or a known phase other than an integer multiple of 180degrees.

In example implementations, a multiplexing technique is applied todetermine the first and second amplitude responses of an interferometricsignal produced by the interferometric sensor at the first and secondpredetermined wavelengths, respectively.

In example implementations, the interferometric signal produced by theinterferometric sensor at the first predetermined wavelength is bandpassfiltered in the optical spectrum to pass the first amplitude response ofan interferometric signal produced by the interferometric sensor at thefirst predetermined wavelength. The interferometric signal produced bythe interferometric sensor at the second predetermined wavelength isalso bandpass filtered to pass the second amplitude response of aninterferometric signal produced by the interferometric sensor at thesecond predetermined wavelength.

Example interferometric sensors include a Fabry-Perot sensor, anExtrinsic Fabry-Perot Interferometer (EFPI) sensor, a Mach-Zendersensor, or a Michelson sensor.

Other example embodiments apply the methods and apparatus to multipleinterferometric sensors.

In example implementations, a first light source may provide light atthe first predetermined wavelength, and a second different light sourcemay provide light at the second predetermined wavelength.

In example implementations, the first operational mode is returned tofrom the second operational mode to determine an absolute measurementreference to be used in a subsequent return to the second operationalmode.

In example applications, the current sensor measurand may be displayedand/or the current absolute optical path length that equates to acurrent sensor measurand value or from which a current sensor measurandvalue is determined may be transmitted to another device.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a schematic block diagram of a interferometric sensorreadout system arrangement;

FIG. 2 shows an example construction of an EFPI type interferometricsensor useable for example as a strain gauge;

FIGS. 3A and 3B are intensity v. optical frequency patterns fordifferent gap lengths of the EFPI sensor example shown in FIG. 2;

FIG. 4 shows detector electrical amplitude output as a function ofsensor gap displacement for a narrow wavelength optical source used inthe readout system depicted in FIG. 1;

FIG. 5 magnifies a portion of the sinusoidal output from FIG. 4 with theinterferometer phase change mapped to sensor gap displacement, with a πphase change being equivalent to a gap change of ¼ of a wavelength;

FIG. 6 is a graph illustrating directional ambiguity in the singlewavelength system detected output shown in FIG. 4;

FIG. 7 graphs the detector amplitude for a narrow laser source as afunction of sensor gap displacement at two wavelengths chosen to samplethe interferometer fringe phase at 90 degrees apart;

FIG. 8 is a graph illustrating the phase lead/lag relationship for twowavelengths at quadrature phase if the rate of change of the sensor gapflips sign;

FIG. 9 is a diagram illustrating a non-limiting example of amulti-wavelength, multi-operational mode interferometric sensing systemfor obtaining during low speed mode accurate absolute sensormeasurements and during high speed mode relative sensor measurements;

FIG. 10 is a diagram illustrating a non-limiting example of amulti-wavelength, multi-operational mode interferometric sensing systemfor obtaining during low speed mode accurate absolute sensormeasurements and during high speed mode relative sensor measurements formultiple interferometric sensors;

FIG. 11 is a flow chart diagram illustrating example interrogationalgorithm steps for implementing a multi-wavelength, multi-operationalmode interferometric sensing in accordance with example embodiments;

FIG. 12 is a graph showing an example interference fringe patternassociated with an example EFPI sensor using a tunable laser swept overa range of optical frequencies/wavelengths;

FIG. 13 is a graph showing a linear region around a quadrature pointassociated with operation of the tunable laser in the high speed mode ofoperation;

FIG. 14 is a graph showing an example of DC data (absolute amplitudemeasurement data) detected during the low speed wavelength scanningmode;

FIG. 15 is a graph showing an example of AC data (relative amplitudemeasurement data) detected during the high speed, dual fixed wavelengthmode; and

FIG. 16 is a graph showing an example where relative AC measurements ofthe sensor gap are referenced to baseline absolute DC measurements.

DETAILED DESCRIPTION

The following description sets forth specific details, such asparticular embodiments for purposes of explanation and not limitation.It will be appreciated by one skilled in the art that other embodimentsmay be employed apart from these specific details. In some instances,detailed descriptions of well known methods, nodes, interfaces,circuits, and devices are omitted so as not obscure the description withunnecessary detail. Those skilled in the art will appreciate that thefunctions described may be implemented in one or more nodes usingoptical components, electronic components, hardware circuitry (e.g.,analog and/or discrete logic gates interconnected to perform aspecialized function, ASICs, PLAs, etc.), and/or using software programsand data in conjunction with one or more digital microprocessors orgeneral purpose computers. Moreover, certain aspects of the technologymay additionally be considered to be embodied entirely within any formof computer-readable memory, such as, for example, solid-state memory,magnetic disk, optical disk, etc containing an appropriate set ofcomputer instructions that may be executed by a processor to carry outthe techniques described herein.

The term “electrical signal” is used herein to encompass any signal thattransfers information from one position or region to another in anelectrical, electronic, electromagnetic, optical, or magnetic form.Electrical signals may be conducted from one position or region toanother by electrical, optical, or magnetic conductors including viawaveguides, but the broad scope of electrical signals also includeslight and other electromagnetic forms of signals (e.g., infrared, radio,etc.) and other signals transferred through non-conductive regions dueto electrical, electronic, electromagnetic, or magnetic effects, e.g.,wirelessly. In general, the broad category of electrical signalsincludes both analog and digital signals and both wired and wirelessmediums. An analog electrical signal includes information in the form ofa continuously variable physical quantity, such as voltage; a digitalelectrical signal, in contrast, includes information in the form ofdiscrete values of a physical characteristic, which could also be, forexample, voltage.

Unless the context indicates otherwise, the terms “circuitry” and“circuit” refer to structures in which one or more electronic componentshave sufficient electrical connections to operate together or in arelated manner. In some instances, an item of circuitry can include morethan one circuit. A “processor” is a collection of electrical circuitsthat may be termed as a processing circuit or processing circuitry andmay sometimes include hardware and software components. In this context,software refers to stored or transmitted data that controls operation ofthe processor or that is accessed by the processor while operating, andhardware refers to components that store, transmit, and operate on thedata. The distinction between software and hardware is not alwaysclear-cut, however, because some components share characteristics ofboth. A given processor-implemented software component can often bereplaced by an equivalent hardware component without significantlychanging operation of circuitry, and a given hardware component cansimilarly be replaced by equivalent processor operations controlled bysoftware.

Hardware implementations of certain aspects may include or encompass,without limitation, digital signal processor (DSP) hardware, a reducedinstruction set processor, hardware (e.g., digital or analog) circuitryincluding but not limited to application specific integrated circuit(s)(ASIC) and/or field programmable gate array(s) (FPGA(s)), and (whereappropriate) state machines capable of performing such functions.

Circuitry can be described structurally based on its configuredoperation or other characteristics. For example, circuitry that isconfigured to perform control operations is sometimes referred to hereinas control circuitry and circuitry that is configured to performprocessing operations is sometimes referred to herein as processingcircuitry.

In terms of computer implementation, a computer is generally understoodto comprise one or more processors or one or more controllers, and theterms computer, processor, and controller may be employedinterchangeably. When provided by a computer, processor, or controller,the functions may be provided by a single dedicated computer orprocessor or controller, by a single shared computer or processor orcontroller, or by a plurality of individual computers or processors orcontrollers, some of which may be shared or distributed.

Optical path length is the product of the geometric length of the paththat light follows through a system and the index of refraction of themedium through which it propagates. A difference in optical path lengthbetween two light paths is often called the optical path difference(OPD). Optical path length is important because it determines the phaseof the light and governs interference and diffraction of light as itpropagates. More specifically, if a light wave is traveling throughseveral different media, then the optical path length of each medium canbe added to find the total optical path length. The optical pathdifference between the paths taken by two optical waves of the samefrequency can then be used to find the phase difference that determineshow the two waves will interfere. Typically, absolute optical pathlength measurements may be used to measure or sense small displacements,but larger displacements may also be measured.

An interferometric sensor has an optical path length that variesdepending on one or more physical parameters to be measured using theinterferometric sensor. Non-limiting examples of an interferometricsensor include a Fabry-Perot sensor, an Extrinsic Fabry-PerotInterferometer (EFPI), such as the example shown in FIG. 2, aMach-Zender sensor, or a Michelson sensor. For ease of description, anEFPI sensor is used in the description below. However, the technologymay be used with other suitable interferometric sensors.

The optical path length associated with the interferometric sensor gapcan be determined based on measured detector amplitude responses. Ameasurement associated with the one or more physical parameters (e.g.,strain, displacement, pressure, temperature, etc.) is then providedbased on the determined optical path length.

FIG. 4 graphs the single detected output of a single wavelength sensorsystem shown conceptually as a sinusoidal transfer function. As the gapincreases, the detected light intensity or amplitude varies.Disadvantages of a single wavelength approach include a non-lineartransfer function and directional ambiguity of the sinusoidal output. Asshown in the graph in FIG. 5, if the sensor gap (½ the Optical PathLength, OPL) changes by a small amount when the initial interferometerphase is a multiple of π (at a peak or valley in the sinusoid), thenthat change in detector amplitude may not be detected because the slopeof the transfer function is small near those points. Additionally, ifthe detector amplitude is near a peak or valley, further amplitudechange from those points may be due either to a decreasing or increasingsensor gap, so there is ambiguity in the direction of sensor gap change.Also, a single wavelength sensor system only detects changes from anunknown starting sensor gap, and accordingly, only provides relativemeasurements of gap length. No absolute measurement of gap length isprovided.

The graph in FIG. 6 illustrates an example of ambiguity in the absolutegap length. A normalized detected interferometric intensity for a singlewavelength (1440 nm) is exactly the same (0.2) for two different gapslengths G1 and G2 which correspond to two different optical pathlengths. As explained above, the optical path length (OPL) of the sensor13 is proportional to the length of the gap 24 multiplied by the indexof refraction for the gap material. When the gap is an air gap, theindex of fraction is 1.0, and the optical path length is twice thelength of the gap length. So the two different gaps G1 and G2 as well astheir respective different OPLs are indistinguishable. Another drawbackof a single wavelength approach is that detector amplitude may vary forreasons other than changes to the sensor gap length, includingvariations in optical attenuation of the optical components andvariations in the detector signal response gain, etc.

In short, although single wavelength systems can be made inexpensively,they are linear over only a short range, can generate multiple differentOPLs for the same intensity, cannot separate intensity variations due toimperfections in the system optics and electronics vs. sensor changes,and provide only relative measurements. Relative measurement systemsneed a known starting condition for the sensor and must makemeasurements faster than the sensor's physical response to be able totrack changes from the starting optical path length.

A two wavelength demodulation system having two quadrature opticchannels may be used to provide unambiguous gap length changemeasurements. The output wavelengths λ₁, λ₂ of two lasers are selectedto generate quadrature, phase-shifted signals for a given sensor airgap. Quadrature signals are ninety degrees out of phase. Consequently,when one of the quadrature signals is at a peak or valley, the otherquadrature signal is in the linear region as shown in FIG. 7. An outputshowing the phase lead/lag relationship at a change in the sign (ordirection) of the rate of change of the sensor gap length, orturnaround, is shown in FIG. 8. Interferometer phase can unambiguouslybe determined by calculating the arctangent of the quadrature signals.Further, the phase shift between signals need not be exactly 90 degreesto unambiguously determine the interferometer phase if the phase offsetis known and it is not an integer multiple of 180 degrees. Although thedual wavelength demodulation system provides a larger linear range, itstill suffers from a drawback noted above for the single wavelengthdemodulation system: that gap length measurements are relative, i.e.,not absolute.

The inventors developed an interferometric sensing system that achieveshighly accurate absolute sensor measurements while also producing rapidrelative sensor measurements. Example embodiments use a fixed-wavelengthsource and a tunable wavelength source and employs two operating modes:(1) a low-speed, swept wavelength mode that provides absolute spectralmeasurements from an interferometric sensor, and (2) a high-speed,quadrature-locked fixed wavelength mode that provides high-speedrelative measurements from the interferometric sensor. Using the twomodes allows the system to make high-speed relative measurements whichare referenced, e.g., periodically, against an absolute measurement. Thesystem provides high-speed sensor interrogation that can also mitigateand/or recover from errors associated with simpler amplitude-basedinterrogation schemes. Example embodiments use a tunable laser as aquadrature-locked source to further increase the range and robustness ofthe high-speed interferometric sensing technology.

In an example embodiments of the swept interrogation mode of operation,a tunable laser is swept across a range of different optical frequenciesin order to make an absolute, low-speed (low bandwidth) measurement.Light reflected from the sensor is collected and converted from the timedomain to the optical frequency domain based on the known rate of thelaser sweep. This sweep produces a “picture” of the sensor's wavelengthdependent reflection spectrum that is then converted to a measurement ofsensor interferometer optical path length by evaluating the opticalfrequencies at which the interference pattern peaks and troughs exist.Because the laser sweeps across multiple interference fringes, itproduces enough data to create an absolute interference lengthmeasurement. Since the laser sweep takes time, e.g., on the order of 10ms using one example tunable laser, the low-speed, swept wavelength modeis used initially and then intermittently, e.g., periodically, whenneeded to reset the absolute interference length measurement reference.

After completing a sweep, the system leaves the low-speed, sweptwavelength mode and switches to the high-speed, quadrature-locked fixedwavelength mode. To produce a high-speed measurement, a fixed wavelengthλ₁ is generated, and the tunable laser 22 is set to a quadraturewavelength λ₂ (i.e., λ₂ is out of phase with λ₁ by 90 degrees) in thesensor's interference pattern selected using information from theprevious full-spectrum laser sweep. The quadrature wavelength λ₂ isdetermined analytically, e.g., using an algorithm implemented by one ormore computers and/or other electronic processing circuitry, usingphysical equations/model of an EFPI sensor (set forth below) and theknown wavelength λ₁ of the fixed wavelength source 20. The amplitude ofthis quadrature signal λ₂ is monitored along with the amplitude of thesignal associated with λ₁ to produce a high-speed, dual-wavelengthsensor interference length measurement.

This high-speed data is fed back to the tunable laser to keep theinterrogator locked at the quadrature point of the sensor. Use of thelaser's tuning mechanism to lock the interrogator in quadratureincreases the useful range of the interferometric sensor and reduces theneed to make frequent low-speed swept measurements in the low speed modeof operation.

In addition to selection of the initial quadrature point, thefull-spectrum sweep data is also used to define the starting positionthat produces a relative interferometric sensor measurement that isreferenced against the absolute interferometric sensor measurement.

To perform a referencing step where an absolute interference fringephase reference is determined, the system switches out of the high-speedquadrature-locked mode and back into the low speed, swept wavelengthmode to perform another laser sweep, e.g., at predetermined intervals oron demand. The timing of this referencing step may be based on anobserved rate of the change of the OPL measurements which is directlyproportional to the external stimuli of the sensor to select a time whenthe sensor's interference length is not rapidly changing. Each time thesystem makes a wavelength sweep, the resulting absolute measurement datais used to (a) reset the initial quadrature point for the high-speedmeasurement mode, and (b) provide an absolute interference fringe phasereference for subsequent dual-wavelength (quadrature-locked)measurements.

An example embodiment of the multi-mode interrogation system is nowdescribed in conjunction with FIG. 9 for an example singlesensor/channel case. The sensor 32 is shown as a device under test (DUT)in the figure. A first laser 20 generates light a single wavelength λ₁,and a second tunable laser 22 generates a swept range of wavelengths inthe low speed mode and at a wavelength λ₂ in the high speed mode ofoperation. The data acquisition and processing circuitry 43 controls themode of operation and the outputs from the lasers 20 and 22. The tunablelaser 22 has a known wavelength profile as a function of time for eachsweep. During the high speed mode, an output the laser 20 at wavelengthλ₁ combined with the tunable laser's single wavelength output λ₂ using a2×2 coupler 26 and routed to the sensor via an optical circulator 30.Light reflected from the sensor 32 travels back to the circulator 30 andis routed to a 1×3 optical splitter 34. The splitter 34 divides thespectral content of the light equally between three output channels eachhaving a respective photodiode light detector 38, 40, and 42. The lightincident on detector 42 is evaluated in the data acquisition andprocessing circuitry 43 during the low speed, swept mode, (sometimesreferred to below as a DC mode). While the tunable laser 22 is sweepingin the low speed mode, the single wavelength laser 20 is deactivated orshuttered either manually or automatically by the data acquisition andprocessing circuitry 43. The light incident on detectors 38 and 40 isprovided for analysis by the data acquisition and processing circuitry43 during the high speed, quadrature mode (sometimes referred to as anAC mode), during which time the data acquisition and processingcircuitry 43 operates the tunable laser 22 to generate afixed-wavelength λ₂ and the fixed wavelength laser 20 to generate thefixed-wavelength λ₁. Alternatively, a tunable laser may be used togenerate the fixed-wavelength λ₁. Although two fixed wavelengths aredescribed here, three or more fixed wavelengths may also be used.Bandpass filters 36 and 38 between the splitter 34 and the detectors 38and 40 ensure that the sensor return signals associated with each of thelasers is passed through to its corresponding detector. Alternatively, amultiplexing technique may be used rather than filtering. For example,the two different wavelength signals may be time multiplexed.Alternatively, a high speed modulation may be applied to each wavelengthsignal and then they are processed using frequency multiplexing. Thedata acquisition and processing circuitry 43 processes the detectedintensities in accordance with suitable interferometric analysis andprocessing algorithms (examples of which are explained below), andpreferably provides a sensor output to a display, a storage device, acomputing device, etc. The sensor output may include the optical pathlength or an engineering unit such as strain, pressure, temperature,etc.

The system shown in FIG. 9 for one sensor/channel may be expanded toaccommodate multiple sensors and channels. An example system for foursensors 341-344 is shown in FIG. 10. Similar reference numbers are usedwith subscripts 1-4 being used identify one of the foursensors/channels. Because the output of both lasers 20 and 22 is sharedamong the four channels, the expansion of the system does not incuradditional cost associated with additional lasers. Each of the fourchannels is similar to the single channel case shown in FIG. 9 and usesfilters and detectors so that each channel may be interrogatedsimultaneously.

Specific example details of interferometric analysis and processing asperformed by the data acquisition and processing circuitry 43 for anEFPI sensor like the one shown in FIG. 2 are now described in detail.The intensity, I, of the return signal reflected from theinterferometric sensor may be expressed as Equation (1) when thereflected signal from the interferometric sensor can be modeled as a2-beam interferometer (in this case, a very low finesse Fabry-Perotcavity). This interference pattern is a function of a series (n=1, 2, .. . N) of wavenumbers, k_(n),I(k _(n))=AI _(o)(k _(n))+BI _(o)(k _(n))cos(k _(n) L+ϕ(k _(n) ,L))+v(k_(n))  (1)where I_(o)(k_(n)) is the source light spectrum, v(k_(n)) is noise inthe measurement, AI_(o)(k_(n)) is the background source spectrum,BI_(o)(k_(n)) is the amplitude modulation on the cosine function, ϕ is awavelength-independent phase term acquired as the signal travels in theinterferometer sensor cavity, and L=2d, where d is the geometricaldistance between the two reflectors (the gap distance) and L is theoptical path length. Calculating optical path length L requiresdetermining the argument of the cosine term, and this requires both anaccurate mathematical model of the argument and sufficient measurementdata. The more interference fringes the sensor return signal contains,the more accurate the calculation of optical path length L. Moreinterference fringes are generated when the illuminating sourcepossesses a wider wavelength range (Δλ). The number of fringes, m, alsoincreases with an increasing gap length according to the formula:

$\begin{matrix}{m = {\frac{\Delta\;\lambda}{\lambda_{1}\lambda_{2}}{L.}}} & (2)\end{matrix}$

When a broadband source is used, the signal returned by theinterferometric sensor is processed using a high resolution spectrumanalyzer to determine the intensity at each wavelength. As the intensityfeatures a sinusoidal modulation, it is commonly referred to as a fringepattern. A more effective option for certain wavelength ranges is to usea tunable laser which quickly sweeps over a set wavelength range. As thelaser sweeps, intensity data is collected as a function of time. Astime, in the case of a swept laser, relates directly to the laserwavelength, this intensity versus time data provides the same fringepattern for analysis as does the output of the spectrum analyzer in thecase of a broadband source.

A flowchart that sets forth an example algorithm for interferometricsensor interrogation using a system such as the examples shown in FIGS.9 and/or 10 based on equation (1) is shown in FIG. 11 and now described.In a first operational mode, light is generated at different scannedwavelengths over a range of wavelengths and provided to theinterferometric sensor (step S1). An amplitude response of aninterferometric signal produced by the interferometric sensor ismeasured over the range of scanned wavelengths (step S2). An absolutemeasurement of an optical path length associated with theinterferometric sensor is determined at each of the scanned wavelengthsbased on the measured amplitude response at each of the scannedwavelengths (step S3). The optical path length varies depending on oneor more physical parameters to be measured using the interferometricsensor. In a second operational mode, light at a first predeterminedwavelength λ₁ and at a second different predetermined wavelength λ₂ isprovided to the interferometric sensor (step S4). The first and secondwavelengths are chosen such that there is a predetermined difference inthe sensor interference fringe phase at the first and secondwavelengths. A first amplitude response of an interferometric signalproduced by the interferometric sensor at the first predeterminedwavelength λ₁ is measured (step S5). A second amplitude response of aninterferometric signal produced by the interferometric sensor at thesecond predetermined wavelength λ₂ is measured (step S6). A relativeoptical path length change is determined based on the amplituderesponses at the first and second wavelengths (step S7). The absoluteoptical path length is combined with the relative optical path lengthchange to determine current absolute optical path length (step S8). Asignal corresponding to the current absolute optical path length isgenerated that equates to a current sensor measurand value or from whicha current sensor measurand value is determined (step S9).

In a non-limiting example implementation, a Phoenix 1200 scanning laser(produced and sold by Luna Innovations, Inc.) was used that is capableof sweeping at rates greater than 5000 nm/s over a 40 nm range in thewavelength range centered at 1540 nm. For the case of a 40 nm scanrange, minimum and maximum wavelengths 1520 nm and 1560 nm, and anarbitrary distance (d) between the input/output fiber and the reflectivetarget of 250 microns (which corresponds to a 500 micron total traveldistance (L)), slightly over 8 fringes were produced. This is sufficientfor the absolute portion of the example algorithm shown in FIG. 11 anddescribed above.

FIG. 12 shows the fringe pattern obtained for an EFPI sensor scannedusing the Phoenix scanning laser at a constant output power. (See S1 andS2 in FIG. 11). The absolute measurement of OPL can be determined fromthe frequency of this detected fringe pattern. (See S3 in FIG. 11).

A Fourier transform method and a least squares sine fit method appliedto the detected intensity data represented in FIG. 12 may be used todetermine an absolute optical path length value L as a reference tomaximize the accuracy of the low speed mode interferometric sensormeasurements. These methods theoretically produce results of equalaccuracy when certain conditions are met, but one or the other mayproduce better results in less time depending on actual conditions. Inan example implementation, the Fourier transform method is used toobtain a good estimate L, which is then used as a seed value for thesine fit method. The accuracy of L calculated by the sine fit methodimproves with better seed values. (This may be implemented, e.g., at S3FIG. 11).

The Fourier transform method has several challenges. One challenge is atoggle, or jump, on the order of half of the center wavelength of theoptical source used. The jump is a result of accumulated error in thealgorithm with respect to identifying the maximum peak location whentaking a fast Fourier transform (FFT) of the fringe pattern.

The Fourier transform method decomposes the signal into frequency (or inthe case of this analysis, from the optical frequency domain totime-of-flight delay domain which can be scaled to optical path length)components. By virtue of the cosine term in Equation (2), one of thosefrequency components occurs at frequency (delay) +L and another at −L.The first term of Equation (2) is transformed to a frequency componentthat coincides with the delay origin. The value of L is determined byfinding the delay position of the peak amplitude of that component.Aliasing effects can create error in the results, but the signal can belowpass-filtered to mitigate this. Windowing, which is a part of theprocess of transforming the signal into the delay domain, alsocontributes error; it causes spectral leakage which can perturb thecontents of nearby frequency bins. Fourier analysis provides directinformation about the frequency content at a number of discretefrequencies (delays); however, the frequency of interest may fall inbetween these frequency points. In that case, interpolation, performedby zero-padding the spectral domain signal before taking the Fouriertransform, combined with peak fitting techniques can be used to estimatethe frequency (delay) of the peak.

Implementing the Fourier transform method begins with windowing thespectral domain interferometric sensor data using a Hamming window orother appropriate window. These data may then preferably be zero padded,e.g., on the order of 2¹²-2¹⁶, resulting in interpolation in thefrequency (delay) domain representation of the signal. Interpolationaids in identifying the frequency (and the associated optical pathlength L) at which the maximum of the peak of interest occurs. Aftertransforming the zero padded spectral signal into the frequency (delay)domain, the frequency of the peak may be roughly identified by searchingfor maximum intensity values around the expected frequency (delay). Theactual frequency of the peak, L, may then be more accurately estimatedby applying a peak-finding technique, such as parabolic interpolation,to the interpolated data.

The least squares sine fit method analyzes the detected signal in theoptical frequency domain. The detected sensor intensity signal isassumed to have the form of Equation (1), and the parameters needed tofit the function generated by Equation (1) to the detected signal asevaluated by the residuals of a least squares fit are determined throughan iterative process. One of the fit parameters is L, and L isdetermined in the process of calculating the fit. Accurate and rapidresults may be achieved using reasonable initial estimates for theunknown parameters (frequency L, offset A, amplitude B, and phase ϕ) inEquation (1). Errors in these initial estimates and noise correlatedwith the input, which can result from an imperfect sensor, can bias theresult. Least squares analysis works well when the form of the inputsignal is well known, and when the system of interest is well understoodand accurately modeled. The least squares method can also fit a varietyof non-linear models and performs well in detecting the frequencyresponse of a system, especially when the frequency of interest isknown.

The least squares method prefers a good seed value for L, which isobtained by first processing the interferometric sensor return signalbased on the Fourier transform method described previously. Depending onthe quality of the raw signal detected, it may also be preferably tofilter the signal.

The spectral domain representation of the signal,x _(n) = A _(n) cos(k _(n) L+ϕ(k _(n) ,L))+ν_(n) ,  (3)includes the amplitude, A_(n) , the gap length L, the phase ϕ, and theresidual noise, ν_(n) , as the unknowns. By defining α=A sin(ϕ) and β=Acos(ϕ), retaining the wavelength-dependent terms in the arguments, andtaking the residual noise and wavelength-independent terms to beconstant, C, Equation (3) can be expressed asy _(n)[α,β,C,L]=α cos(k _(n) L+ϕ(k _(n) ,L))+β sin(k _(n) L+ϕ(k _(n),L))+C  (4)An iterative least squares estimate technique is used to determine L towithin a desired accuracy without requiring values of parameters α, β,and C to be determined for many applications. Equation (4) may beexpressed asy=Dx  (5)where the y values are the measured data, andx=(α,β,C)^(T).  (6)The D matrix contains the parameter of interest, L

$\begin{matrix}{D = \begin{matrix}{\cos\mspace{11mu}\left( {{k_{1}L} + {\phi\left( {k_{n\; 1},L} \right)}} \right)} & {\sin\mspace{11mu}\left( {{k_{1}L} + {\phi\left( {k_{n\; 1},L} \right)}} \right)} & 1 \\\vdots & \vdots & \vdots \\{\cos\mspace{11mu}\left( {{k_{N}L} + {\phi\left( {k_{N},L} \right)}} \right)} & {\sin\mspace{11mu}\left( {{k_{N}L} + {\phi\left( {k_{N},L} \right)}} \right)} & 1\end{matrix}} & (7)\end{matrix}$

A grid of L_(i) (with i=1, 2, . . . , M), and with the seed value of Las the mean, is generated, and then the matrix D_(i) is calculated foreach L_(i). Using that value of D_(i),ĝ _(i) =y ^(T) D(D ^(T) D)⁻¹ D ^(T) y  (8)is evaluated. The value of L_(i) that corresponds to the maximum valueof gi is the best estimate of L. This process is repeated to refine theestimate of the absolute measurement for L in the low-speed operationalmode. (See S3 in FIG. 11).

Interrogating a low finesse Fabry-Perot cavity with a single wavelengthsource in the high speed operational mode returns a sensor intensitysignal that can be expressed as:I _(r) =|A ₁ +A ₂|² ≅A ₁ ² +A ₂ ²+2A ₁ A ₂ cos(kL+ϕ(L,k))  (9)where A₁ and A₂ are the amplitudes of the signals reflected from theinput/output fiber end face and the reflective target, respectively,k=2π/λ is the wave number, L is twice the separation between fiber andtarget, and ϕ is an additional phase difference acquired by the A₂ beamas it travels in the cavity. The gap length is determined by monitoringthe intensity of the reflected beam and using that data to determine thephase of the cosine argument. (See S4 in FIG. 11). At a fringe maximum,

$\begin{matrix}{L = {\frac{\lambda}{2\pi}{\phi\left( {L,k} \right)}}} & (10)\end{matrix}$

Using a single wavelength interrogation approach and assuming amonotonously changing gap length, it is possible to determine the gaplength, referenced to some DC baseline value, by monitoring theintensity signal and counting fringes. However, if the change in gaplength switches from increasing to decreasing (or vice versa) at afringe maximum or minimum, the reversal of direction cannot be detected.This single-wavelength approach can result in directional ambiguitywhich can have a substantial impact on the accuracy of the gapmeasurement. In addition, the sinusoidal function is nonlinear, and itoffers a lesser sensitivity when the intensity pattern falls in the peakand valley regions than when it is centered on the linear portion of thecurve. This can be seen in the example graph in FIG. 13.

One example way to ensure that the direction of gap change is known andto make the resolution along the entire sinusoidal transform functionmore uniform is to use a 2-wavelength (λ₁, λ₂) interrogation approachcorresponding to the above-described high-speed mode of operation. (SeeS5 and S6 in FIG. 11). This approach functions well when the returnsignals corresponding to the two wavelengths are in quadrature: when thesensor is illuminated by the second wavelength, the phase of the cosineterm in Equation (11) is different by an odd multiple of π/2 than whenit is illuminated by the first wavelength. Mathematically, this isexpressed

$\begin{matrix}{{{{L\left( \frac{2\pi}{\lambda_{2}} \right)} + {\phi\left( {L,\frac{2\pi}{\lambda_{2}}} \right)} - {L\left( \frac{2\pi}{\lambda_{1}} \right)} - {\phi\left( {L,\frac{2\pi}{\lambda_{1}}} \right)}} = {\frac{\pi}{2}\left( {{2m} + 1} \right)}},} & (11)\end{matrix}$where m=1, 2, . . . and it can be seen from this equation that operationin quadrature depends not only on the interrogating wavelengths, but onthe gap length as well. As shown in FIG. 13, the quadrature point neednot be located on the same fringe as the reference point.

When the system is operated in quadrature, the gap separation may beobtained from the normalized amplitudes of the reflected signalintensities,

$\begin{matrix}{{\left( {k_{1},L} \right)} = {\cos\mspace{11mu}\left( {{k_{1}L} + {\phi\left( {k_{1},L} \right)}} \right)}} & (12) \\\begin{matrix}{{\left( {k_{2},L} \right)} = {\cos\left( {{k_{2}L} + {\phi\left( {k_{2},L} \right)} + {\phi_{12}(L)}} \right)}} \\{= {\cos\mspace{11mu}\left( {{k_{1}L} + \frac{\pi}{2} + {\phi\left( {k_{1},L} \right)} + {\phi_{12}(L)}} \right)}} \\{{= {\sin\mspace{11mu}\left( {{k_{1}L} + {\phi\left( {k_{1},L} \right)} + {\phi_{12}(L)}} \right)}},}\end{matrix} & (13)\end{matrix}$where ϕ₁₂ is the gap-dependent phase deviation from quadrature. When ϕ₁₂is zero,

k 1 ⁢ L + ϕ ⁡ ( k 1 , L ) = atan ⁢ ± m ⁢ ⁢ π , ( 14 )and when ϕ₁₂ is non-zero a correction derived from basic trigonometricidentities will be used.

Example embodiments optimize this mathematical model used to describethe function of the return signal's intensity. An optimization processincludes investigating whether the sensor return signal is moreaccurately modeled as Fabry-Perot cavity rather than a low finesseFabry-Perot cavity. The above description involving equations 1-14assumed a low finesse Fabry-Perot cavity, which is modeled by Equation(1). In contrast, the transfer function of a Fabry-Perot cavity is:

$\begin{matrix}{{I_{r}\left( {k_{n},L} \right)} = \frac{{1 - {\cos\mspace{11mu}\left( {{k_{n}L} + {\phi\left( {k_{n},L} \right)}} \right)}}\mspace{11mu}}{1 + R^{2} - {2R\mspace{11mu}\cos\mspace{11mu}\left( {{k_{n}L} + {\phi\left( {k_{N},L} \right)}} \right)}}} & (15)\end{matrix}$where R is the power reflectivity of the optical interfaces. Thedenominator of Equation (15) is approximately equal to 1 in the case ofa very low finesse Fabry-Perot cavity, which yields Equation (1).

Once the intensities are known (i.e., detected), they may be combinedand used to determine the relative change in the OPL of the sensor (seeS7 in FIG. 11) from the previous measurement.

By adding the relative OPL change of the first measurement in the secondmode of operation to the absolute measurement determined (see S3 in FIG.11), the absolute OPL is determined from the combination of the twomodes. Building upon this for subsequent measurements (see S8 in FIG.11), the absolute OPL may be tracked.

Knowledge of the absolute OPL enables a user to generate a signal thatequates to the sensor's measurand through an appropriate conversionwhich may be obtained when the sensor was designed or calibrated (see S9in FIG. 11).

Accordingly, the example sensor measurement and interrogation systems,such as the examples shown in FIGS. 9 and 10, combine two interrogationalgorithm approaches where a tunable laser operates in a first low speedmode to produce an absolute measurement system to track the optical pathlength (which can be related to a desired sensor measurand) when it ischanging slowly (which is why the label DC signal is used), and twofixed wavelength lasers operated in the high-speed, relative-measurementmode to make measurements when the signal is changing quickly (which iswhy the label AC signal is used). The relative measurements are madewith respect to the last absolute measurement, and an absolutemeasurement may be made at any time to recalibrate the reference value.

Using a tunable laser 22 that scans over a desired wavelength range andthat can also be operated at a fixed and user-specified wavelength λ₂provides accuracy and flexibility. The quadrature condition depends onthe gap width of the EFPI sensor. The ability to customize the tunablelaser's fixed-wavelength emission means that the optical system can beoptimized to be in quadrature for each EFPI sensor gap without having toreplace optical network components such as the filters which prevent thefixed wavelength signal from interfering with the swept wavelengthwithin the designed range. This also enables the data acquisition andelectronic processing circuitry 43 to actively adjust the emission fromthe tunable laser 22 in order to remain in quadrature as the EFPIsensor's gap length increases or decreases.

Depending on the application, the combined absolute and relative sensormeasurement technology may be limited by the speed of the dataacquisition and electronic processing circuitry 43 used to process thesignal. Faster processing may be more desirable in real timeapplications. The technology enables a low-speed sensor reading to bedetermined initially and then a dynamic, high-speed measurement tosubsequently be tracked at very high data rates. While the high-speedmeasurement may be relative to the low-speed absolute measurement, it ispossible to determine the absolute measurement value for the sensor bycombining the low-speed and high-speed measurement values. Exampleembodiments support this flexibility to provide accurate data reductionwith little or no jumps and/or ambiguities while calculating therelative measurement of the sensor.

The system was described above using an EFPI sensor. The system may alsobe used in other environments such as an environment that creates aFabry-Perot cavity that can be manipulated accurately through mechanicalmeans. This simulates the changes which would be seen with an appliedstimuli. An EFPI sensor detects an external stimulus (i.e., pressure ortemperature changes) by having a sensor material that responds to thestimulus as a change in the size of the Fabry-Perot cavity. As themeasured parameter changes, the sensor material reduces or increases thesize of the optical gap on the EFPI sensor. The interrogation light thattraverses this air gap interferes with reference light within thesensor. By mechanically varying the size of an air gap, the effects ofan external stimulus can be tested directly without having a test setupthat requires extreme pressures or temperatures.

In one example experiment, one end of an optical fiber was connected tothe data acquisition and electronic processing circuitry 43, and theother end was cleaved to be perpendicular to the long axis of the fiber.The cleaved end of the optical fiber was taped along a groove in amicrometer-positioned stage with the cleaved end face positioned in thegap between the two stages. A reflector fashioned from a short length ofoptical fiber with a cleaved end was taped along the groove in thepiezo-driven stage so that the cleaved end face was also located in thegap between the two stages. The cleaved end faces of the one fiber endand the reflector were aligned such that light emitted from the onefiber end reflected off of the reflector and was collected by the onefiber end. Foam was placed under the base of the setup to effectvibration isolation. A box was placed over the setup during dataacquisition to isolate the setup from air currents.

DC data were taken with the piezo-driven stage held in one position. DCdata measurements include signal data acquired while the tunable laserscans over its full range. The resulting data, which includes signalintensity as a function of laser wavelength, are analyzed, and absolutevalues of the sensor width (the air gap) were calculated from them. Thesystem was then transitioned to AC mode by opening the switch betweenthe fixed wavelength laser (emission wavelength 1530 nm) and the opticalnetwork and locking the tunable laser wavelength at an emissionwavelength in the vicinity of 1551 nm. The exact locked wavelength waschosen by analyzing the last set of DC data and determining whatwavelength was required to place the fixed laser and the locked lasersignals in quadrature with one another.

After transitioning the system to AC mode, AC data were taken while thepiezo-driven stage reduced the gap by 5 microns and was held briefly atthat new position. Thereafter, the gap was increased by 5 microns toreturn to the starting position. The rate of translation of the stagewas 0.25 microns/s. The last DC signal acquired prior to transitioningto AC mode is shown in FIG. 14. Analysis of the fringes in FIG. 14 bythe data acquisition and electronic processing circuitry 43 determined asensor width of 157.873 microns.

Based on the sensor width and the wavelength of the fixed laser, thedata acquisition and electronic processing circuitry 43 calculated thatlocking the tunable laser's wavelength at 1554.8128 nm would put thesignals from the fixed-wavelength and the locked-wavelength lasers inquadrature with one another.

After transitioning to AC mode, the signals produced by both lasers werecollected while the piezo-driven stage reduced the gap by 5 microns,held steady, and then increased the gap by 5 microns to return to itsinitial state. The signals generated during this procedure are shown inFIG. 15.

The AC data are analyzed by the data acquisition and electronicprocessing circuitry 43 as they are acquired to determine change insensor width. The data acquisition and electronic processing circuitry43 only has access to the most recently acquired pair of signal data(one point from the fixed-laser signal and one point from thelocked-wavelength laser) and the previously-taken data. When the changesin the sensor width (calculated from the AC data) were added to theabsolute value of the width (calculated from the DC mode), the values ofsensor width during the piezo-driven stage experiment was determined.The starting gap was determined to be 157.876 microns, the gap after the−5 micron translation to be 152.862 microns, and the final gap to be157.861 microns. The calculated sensor width data were consistent withthe expected movement of the piezo-driven stage.

FIG. 16 shows a graph of calculated sensor width values from the data inFIG. 15 with the relative AC measurement values referenced to baselineabsolute DC measurement values. These data demonstrate some of themerits of the disclosed technology and its ability to measure theabsolute gap change of an EFPI sensor at high speeds with low latency.The control scheme enabling the two wavelengths to remain in quadraturewith the EFPI gap changing over a large range is also advantageous formaking accurate measurements by enabling a new wavelength at which toset the scanning laser for the resulting EFPI gap (OPL) to bedetermined.

Although various embodiments have been shown and described in detail,the claims are not limited to any particular embodiment or example. Noneof the above description should be read as implying that any particularmember, step, range, or function is essential such that it must beincluded in the claims scope. The scope of patented subject matter isdefined only by the claims. The extent of legal protection is defined bythe words recited in the allowed claims and their equivalents. Allstructural and functional equivalents to the members of theabove-described preferred embodiment that are known to those of ordinaryskill in the art are expressly incorporated herein by reference and areintended to be encompassed by the present claims. Moreover, it is notnecessary for a device or method to address each and every problemsought to be solved by the technology described, for it to beencompassed by the present claims. No claim is intended to invokeparagraph 6 of 35 USC § 112 unless the words “means for” or “step for”are used. Furthermore, no embodiment, feature, component, or step inthis specification is intended to be dedicated to the public regardlessof whether the embodiment, feature, component, or step is recited in theclaims.

The invention claimed is:
 1. A measuring method for use with aninterferometric sensor, comprising: operating one or more light sourcesto provide light over a range of source wavelengths including providinglight at three or more wavelengths to the interferometric sensor;determining a period of an interferometric amplitude response of aninterferometric signal produced by the interferometric sensor over therange of source wavelengths; determining an absolute optical path lengthassociated with the interferometric sensor based on the determinedperiod of the interferometric amplitude response of the interferometricsignal over the range of source wavelengths, where the optical pathlength varies depending on one or more physical parameters to bemeasured using the interferometric sensor; providing light at two ormore selected wavelengths to the interferometric sensor, with two of theselected wavelengths having two different relative phases on theinterferometric amplitude response; measuring a first interferometricamplitude response of an interferometric signal produced by theinterferometric sensor at a first of the two or more selectedwavelengths; measuring a second interferometric amplitude response of aninterferometric signal produced by the interferometric sensor at asecond of the two or more selected wavelengths; determining a relativeoptical path length change based on the first interferometric amplituderesponse at the first selected wavelength and the second interferometricamplitude response at the second selected wavelength; combining theabsolute optical path length with the relative optical path lengthchange to determine a current absolute optical path length; andgenerating a signal corresponding to the current absolute optical pathlength that relates to a current sensor measurand value or from which acurrent sensor measurand value is determined.
 2. The method of claim 1,further comprising: using one or more lasers to provide light to theinterferometric sensor including (i) three fixed wavelength lasers, (ii)a fixed wavelength laser and a tunable wavelength laser, or (iii) onetunable wavelength lasers.
 3. The method of claim 2, further comprisingmodulating the one or more lasers to provide time varying illuminationof the interferometric sensor.
 4. The method of claim 3, wherein thetime varying illumination includes changing the illumination atdifferent wavelengths at different times.
 5. The method of claim 4,further comprising: receiving, at a detector, light returning from theinterferometric sensor, and discriminating light at differentwavelengths based on the time at which the light is received.
 6. Themethod of claim 3, wherein the modulating includes modulating the two ormore lasers using one or more waveform signals at different frequencies.7. The method of claim 6, further comprising: receiving, at a detector,light returning from the interferometric sensor, and discriminatinglight at different wavelengths based on the different frequencies of theone or more waveform signals.
 8. The method of claim 2, furthercomprising: using more than two wavelengths of light to illuminate theinterferometric sensor, and selecting two of the more than twowavelengths and determining a relative optical path length change basedon a relative phase of the interferometric amplitude response at thosewavelengths.
 9. The method of claim 1, further comprising: performingthe operating of the one or more light sources, the determining of theperiod of the interferometric amplitude response, and the determining ofthe absolute optical path length in a first mode of operation, andperforming the providing of light at the two or more predeterminedwavelengths to the interferometric sensor, the measuring of the firstand second interferometric amplitude responses, and the determining ofthe relative optical path length change in a second mode of operation,wherein the first and second modes of operation are performed during thesame time period or during different time periods.
 10. The method ofclaim 9, further comprising: correcting information determined in thesecond mode of operation based on information determined during thefirst mode of operation.
 11. The method of claim 1, further comprising:splitting the light to multiple channels to illuminate multipleinterferometric sensors, each of the multiple interferometric sensorshaving an associated light detector.
 12. The method of claim 1, furthercomprising: switching the light between multiple interferometricsensors, and multiplexing a single detector to measure outputs from themultiple sensors.
 13. A system for use with an interferometric sensor,comprising: one or more light sources to provide light over a range ofsource wavelengths including providing light at three or morewavelengths to the interferometric sensor; processing circuitry to:determine a period of an interferometric amplitude response of aninterferometric signal produced by the interferometric sensor over therange of source wavelengths; determine an absolute optical path lengthassociated with the interferometric sensor based on the determinedperiod of the interferometric amplitude response of the interferometricsignal over the range of source wavelengths, where the optical pathlength varies depending on one or more physical parameters to bemeasured using the interferometric sensor; wherein the one or more lightsources provide light at two or more selected wavelengths to theinterferometric sensor, with two of the selected wavelengths having twodifferent relative phases in the interferometric amplitude response;wherein the processing circuitry is configured to: measure a firstinterferometric amplitude response of an interferometric signal producedby the interferometric sensor at a first of the two or more selectedwavelengths; measure a second interferometric amplitude response of aninterferometric signal produced by the interferometric sensor at asecond of the two or more selected wavelengths; determine a relativeoptical path length change based on the first interferometric amplituderesponse at the first selected wavelength and the second interferometricamplitude response at the second selected wavelength; combine theabsolute optical path length with the relative optical path lengthchange to determine a current absolute optical path length; and generatea signal corresponding to the current absolute optical path length thatrelates to a current sensor measurand value or from which a currentsensor measurand value is determined.
 14. The system of claim 13,wherein: the one or more light sources comprises one or more lasers toprovide light to the interferometric sensor, wherein the one or morelasers include at least three fixed wavelength lasers, a fixedwavelength laser and a tunable wavelength laser, or at least one tunablewavelength lasers.
 15. The system of claim 14, wherein the one or morelight sources is adapted for modulation or coupled to or with amodulator to provide time varying illumination of the interferometricsensor.
 16. The system of claim 15, wherein the time varyingillumination includes changing the illumination at different wavelengthsat different times.
 17. The system of claim 16, further comprising: adetector to receive light returning from the interferometric sensor,wherein the processing circuitry is configured to discriminate light atdifferent wavelengths based on the time at which the light is received.18. The system of claim 16, wherein the one or more light sources isadapted for modulation or coupled a modulator to modulate the light atdifferent wavelengths using one or more waveform signals at differentfrequencies.
 19. The system of claim 18, further comprising: a detectorto receive light returning from the interferometric sensor, wherein theprocessing circuitry is configured to discriminate light at differentwavelengths based on the different frequencies of the one or morewaveform signals.
 20. The system of claim 14, wherein the processingcircuitry is configured to: use more than two wavelengths of light toilluminate the interferometric sensor, select two of the more than twowavelengths for processing based on a relative phase of theinterferometric amplitude response at those wavelengths.
 21. The systemof claim 13, wherein the system is configured to: operate the one ormore light sources, determine the period of the interferometricamplitude response, and determine the absolute optical path length in afirst mode of operation, and provide light at the two or morepredetermined wavelengths to the interferometric sensor, measure thefirst and second interferometric amplitude responses, and determine therelative optical path length change in a second mode of operation,wherein the first and second modes of operation are capable of beingperformed during the same time period or during different time periods.22. The system of claim 21, wherein the processing circuitry isconfigured to: correct information determined in the second mode ofoperation based on information determined during the first mode ofoperation.
 23. The system of claim 13, further comprising: a splitter tosplit the light to multiple channels to illuminate multipleinterferometric sensors, each of the multiple interferometric sensorshaving an associated light detector.
 24. The system of claim 13, furthercomprising: one or more switches to switch the light between multipleinterferometric sensors and to multiplex a single detector to measureoutputs from the multiple sensors.